Phase modulation



April 20, 1954 R. c. CORDERMAN 76,303

PHASE;- MODULATION Filed Feb. 19, 1951 n ll 3 I; v

OSCILLATOR SIGNAL f SOURCE AAAA HIV"

AAAIA VIVVI INVE/V TOR y R. C. CORDERMAN A 7' TORNEV circuit embodying Patented Apr. 20, 1954 2,676,303 PHASE MODULATION Roy C.- Corderman, Winston-Salem, N. 0., as-

, signor to Western Electric rated, New York, N.

York

Company, Incorpo- Y., a corporation of New Application February 19, 1951, Serial No. 211,734

Claims. (01. 332-28) This invention relates to signal modulation and more particularly to phase modulation.

It is an object of the invention to augment the phase shift per unit of modulating signal amplitude obtained with a phase modulator.

In accordance with a specific embodiment of the invention disclosed in detail below for purposes of illustration, the phase of a radio frequency wave from a stabilized source is modulated in a conventional manner by an input signal. A portion of the phase modulated wave is then fed back to the input of the phasemodulator and combined with the wave from the stabilized source so that the phase of the radio frequency waves'supplied to the modulator is varied I in proportion to the instantaneous phase modulation. The net result is that the resultant phase shift per unit of modulating signal amplitude is increased. The circuit is a positive feedback arrangement so thatcare is taken to properly limit the feedback voltage to prevent instability. The phase shift of the modulated waves due to the feedback is smaller than the initial phase shift due to the modulating signal so that the action is convergent in nature. For example, in the case of a stepped input signal, the output wave will seek stable phase displacements.

The invention, its features and objects may be better understood from a consideration of the following detailed description when read in accordance with the attached drawings in which:

Fig. 1 shows schematically a phase modulation principles of the present invention; and

Figs. 2, 3, l, and 5 are vector diagrams illustrating voltages at various points in the, circuit at Fig. 1. V

Referring now to Fig. 1 radio frequency waves to be modulated are supplied from a stabilized oscillator H which may, for example, comprise a crystal oscillator of well-known type to the input of a first amplifier l2. The amplifier I 2 comprises a conventional pentode having a tuned plate load I 3 which is tuned to resonance at the frequency of the waves from the source H. Biases are supplied in a conventional manner. shunted-across the plate load I3 is a phase modulator comprising a reactance tube I to which are fed modulating voltages from a signal source l5. The reactance tube I4 is a conventional circuit and comprises a pentode l6 whose plate-cathode circuit is in shunt with the tuned plate circuit I 3 of the amplifier I2. Resistor l1 and condenser l 8 comprise a phase splitter which feeds to the control grid of the pentode I6 a voltage which is 90 degrees out ofphase with the voltage across the tuned circuit l3. The reactance tube l4 acts as a variable reactance whose example in the Radio Engineer's Handbook F; EJTerman, McGraw Hill 1943, at'pages and 654 and 655. The radio frequency waves value is dependent upon the instantaneous amplitude of the voltage supplied by the signal source 15. Being shunted across the tuned circuit l3, it acts to detune circuit I 3 in accordance with the modulating signal amplitude and hence shifts the phase of the" amplified waves from oscillator H which appear in the output circuit of amplifier l2. Reactance tube phase modulators are well known and are described for 'now shifted in phase in accordance with the modulating signals are next applied to amplifier 2| which has a plate load circuit including the two tuned circuits 22 and 23. The parallel tuned circuit 22 is" tuned to resonance at afrequency which is twice the frequency of the'waves sup- "plied by oscillator I! while resonant circuit 23 is tuned to the fundamental. An output circuit 24 is inductively coupled to the resonant circuit 22 which is tuned to the double frequency. Frequency doubling aids in the production of a large modulation index as is well known. In addition to operating as a frequency doubling amplifier,

tube 2! also acts as a limiter by virtue of the grid leak resistor 25. Grid leaklimiter circuits are well known and by proper proportioning of the circuit constants associated with tube 2! both positive and negative limiting may be obtainedat desired values.

,A feedback circuit comprising the variable resistor 28 and condenser 29 applies a portion of thephase modulated voltage of the fundamental "frequency appearing acr'oss tuned circuit 23 to the control grid of amplifier 12.

supplied by the oscillator I 'I I Since the arrangement comprises an even number of stages thefeedback is positive in nature so that care must be taken by adjusting resistor 28, for example, to limit the maximum amplitude of the feed back voltage to prevent instability. The feedback voltagecombines vectorially with the waves v I and thus modifies the phase of the voltage supplied to the modulator in accordance with theinstantaneous phase modulation.

The operation of the circuit may be better understood by referring to the vector diagrams, Figs. 2 through 5. Referring to Fig. 2, vector 0 represents the input voltage to tube l2 supplied by the stabilized oscillator ll. Vectors A represent the amplified voltage appearing at the output of tube l2 with a 180-degree phase shift due to the operation of the amplifier and with a further phase shift, plus or minus, resulting from the modulating potential; the dotted vectors illustrate the range of the phase shift ..resulting from a given input signal. The vectors A represent the amplified phase modulated. voltages-appearingat'the output of amplifier '21. Thevectors B1 which are merely a portion of the larger vectors A represent the initial feedback voltage which is fed back to the input of tube 12;

The vectors of Fig. 3 illustrate the second step in the operational analysis: the oscillator is now combined with-the-modulated feedback voltage Bi voltage C1; this resultant voltage is applied to the input of tube i2. It may not be readily observed that the length of vectors C1 vary in length, but this is true as it is obvious" that the addition of vector with vector B1 in the modulated condition issh'orterthan the addition of vector 0 Wilih VBQ'COY-Bpifl the-unmodulated or restingcondition. Vectors. D representfl'the amplified voltage Ciappearingvatthe output of tube l2'which has .furthernbeen shifted in phase by the:modulating-voltage: supplied by the signal source 15. These voltages are. further amplified by tube 2.! and appear.-attheoutput-of tubezl as vectors D, thevectorsxBe now represent the feedback'voltage.

The vectors :of Fig. 4 merely carry the analysis one .stepfurthen. The .initial'voltage O isnow combined: with" the; feedback :voltageaBz to form the resultant C'z: and" asxamplifiecl; and further shifted ;in:phase by the modulating potential .appearsacross'thettuned ch cuit 13 asvectors E. Vector E 8.116.133 represent-,;respeetively; the yoltages "appearing: :at *the output-ofxtube 2 I "and the feedback-voltage;

Fig-5 illustrates theanaximumrcondition which is permissible; namely,the;conditioniwhere the feedback voltage. Bn. when: addedto the input voltage 0- generates a: combined voltage On which is equal to vector-O..- Th'is-limit cannot be fully reached .as it'is: evident that .thezfeedback-voltage if greater than theoriginal-voltagewould result in instability.

Inthe foregoingdiscussionof :the vector diagrams, it has beenrepresented that successive cycles of modulation in"theifirst'vaouum tube 12 z and amplification in the-finaltubeli occur. It should :be understood, however, thatthese events occur substantially simultaneously; @the; successive changes in the character of thersignal :hav-

ingbeen represented merely to aid ins-analyzing:

the-over-all operation of the circuit. 1

The maximum phase-displacement obtainable with the circuitof Fig. 1 isequalto one radian or approximately filladegrees; This isfiv-e times the ll degree shift which is-. ordinarily considered the maximum permissible :in .a phase modulated tube, and-hence, reduces-by a factor of 5 the multiplicationstages ordinarily necessary to obtain a comparable phase It will be noted that .by virtueot the limiting action provided .bytube 2i and-the associated feedback, distortionnormally present in phase modulated systems when operating .athigh .levels of modulation is eliminated as the limitingv action removes amplitude products. of modulation.

The voltage=0 from to form aresultant Therefore, the lengths of the vectors D and E under the modulated and unmodulated conditions will be substantially the same. By a proper adjustment of the limiting action of tube 2| the output phase displacement of the modulating signal may be made substantially linear with respect to .the applied modulating signal.

Although the invention has been described as relating to a specific embodiment, numerous other embodiments and modifications will readily occur toone skilled in the art so that the invention should not be deemed limited to the specific embodiment enclosed.

' What is-claimedis:

1. A regenerative phase modulation circuit comprising a source of waves to be modulated, arfirst amplifier, means to apply said waves to be modulated to the input of said first amplifier, a'source'of modulating signals, means connected in the output of saidifirst amplifier for modulating' thep'hase 'of said waves inraccordance with said signals, a-seeondamplifier, means-to apply the phase shifted waves' to the: input of said second amplifier, .an output circuit connected to the output of saidsecond amplifier; .and'a feedback circuit to' apply a portion. of the-output of said second amplifier to .the inputofisaid first amplifier.

2. The combinationiin accordancewith claim 1 wherein said secondamplifierzincludes limiter means for limiting theroutputpfthe second amplifier.

3. A modulation circuit comprising: asource ofwaves to be modulated, a source-oi modulating signals, a phase modulator, ,means-forapplying said wavesand said-sisiialsto said modulator, and means for combiningzsa portion. of the output of said modulator with the said waves applied to said modulator: comprising; afeedback circuit connected between theoutput of said modulator and said means foi." applying said waves to said modulator.

4. A 'phase'modulation' circuitcomprising a stabilized source ofrad-io frequency waves, a first amplifier having an output circuit including a circuit tuned to resonance atthe frequency of said waves, means -.to apply said radio frequency waves to theinput of said first amplifier. a reactance tube modulator connected in shunt with said tuned circuit, means-tov apply modulating signal waves to said reactance tube, a second amplifier means to apply the output of said first amplifier to the input-of said second amplifier, an output circuit connected to the output of .said

. second amplifier, andmeans to apply a portion References Cited'in the file of this patent UNITED STATES PATENTS Number Name Date 2,279,660 Crosby Apr. 14, iii i2 2,436,834 Stodola Mar. 2, 1948 2,504,050 Rohde Apr. 11, 1950 2,566,405 DeLange et al. Sept. 4, 1951 2,598,722 Richards June 3,; 1952 

